Crossbar interconnection networks are essential components in a variety of optical signal processing applications, such as communication signal switching and parallel computation. Such applications are described in articles such as that by M. Fukui entitled "Optoelectronic parallel computing system with optical image crossbar switch", in Applied Optics, Vol. 32, pp 6475-6481 (1993), and that by Y. Wu, L. Liu, and Z. Wang, entitled "Optical crossbar elements used for switching networks", published in Applied Optics, Vol. 33, pp. 175-178 (1994). The disclosures of these publications, and of those mentioned in any other part of the specification, and the disclosures of all documents cited in any of the publications, are hereby incorporated by reference.
Such crossbar interconnection networks have been incorporated into optical configurations for performing dynamic vector matrix multiplication and arbitrary interconnection between N inputs and N outputs. They take advantage of the speed and the parallelism of optical signal transmission to provide performance levels significantly better than those attainable using microelectronic devices.
Several such systems, using discrete components and free space propagation, have been described in the literature, such as by J. W. Goodman, A. R. Dias, and L. M. Woody in "Fully parallel, high-speed incoherent optical method for performing discrete Fourier Transforms", published in Optics Letter, Vol. 2, pp. 1-3 (1978); by M. Fukui and K. Kitayama in "High-throughput image crossbar switch that uses a point light source array", published in Optics Letters, Vol. 18, pp. 376-378 (1993); by P. C. Huang, W. E. Stephens, T. C. Banwell and L. A. Reith in "Performance of 4.times.4 optical crossbar switch utilizing acousto-optic deflector" published in Electronics Letters, Vol. 25, pp. 252-253 (1989); by K. H. Brenner and T. M. Merklein in "Implementation of an optical crossbar network based on directional switches", published in Applied Optics, Vol. 31, pp. 2446-2451 (1992); and by R. W. Cohn in "Link analysis of deformable mirror device based optical crossbar switch", published in Optical Engineering, Vol. 31, pp. 134-140 (1992).
Such configurations typically consist of a number of conventional lenses and a dynamic spatial light modulator (SLM). The use of discrete optical components, and the relatively large number thereof required, results in a switch with comparatively high weight and volume. Furthermore, the individual components have to be mounted and aligned mechanically. The alignment accuracy required between individual components is extremely difficult to achieve and often impractical for the high density of channels to be switched. As a result, free space propagation optical crossbar switching systems are very sensitive and unruggedised, have relatively low positioning accuracy and are subject to thermal instability, thus making them unsuitable for general industrial use.
All of these factors combine to make such free space switches incompatible with the small size and circuit construction techniques used in the integrated optoelectronic technology used in modern signal processing and communications system. There is, therefore, need for an optical crossbar switch which combines the speed of optical processing techniques with the small dimensions typical of microelectronic technology.
The problems associated with discrete elements and free space configurations can be alleviated by using planar optics configurations, in which several optical elements (lenses, filters, beam splitters, polarisers, etc.), can be integrated onto a single substrate. The light propagates between the different optical elements, inside the substrate, either by total internal reflection or with the aid of reflective coatings on the substrate surfaces. The alignment of several optical elements integrated onto one substrate can be done with relatively high accuracy during the recording of the elements in the laboratory. Planar optical technology is fully compatible with microelectronic detectors, devices and production technology, with the element patterns being generated by standard microelectronic production techniques such as photolithography and etching.
Such planar optical systems have been developed to perform basic imaging functions, as described by J. Jahns and S. Walker in the article "Imaging with planar optical systems" in Optics Communications, Vol. 76, No. 5-6, May 1990, pp. 313-317. Specific interconnect designs for use in planar optical systems are described by Y. Amitai and J. Goodman in "Design of substrate-mode holographic interconnects with different recording and readout wavelengths", as published in Applied Optics, Vol. 30, pp. 2376-2381 (1991), and by S. H. Song, et al in "Planar optical implementation of crossover interconnects", in Optics Letters, Vol. 17, pp. 1253-1255 (1992). However, no method has been described heretowith whereby an optical crossbar switch can be implemented in planar technology.